In the past, electro-thermal, electro-mechanical, and thermal mechanical structures have been manufactured using unfired, i.e. "green" or "raw", ceramics and glasses which are machined and then fired. These materials have been used because of their mechanical integrity and ability to be incorporated with electrical circuitry. In some cases, they were used because of high temperature resistance. Some of these devices have previously been made of multilayer ceramic construction.
The multilayer ceramic process is widely known. In general, the process consists of forming a slurry of ceramic and glass powders combined with thermoplastic organic binders and solvents. The slurry is doctor-bladed onto a carrier. After volatilization of the high vapor pressure solvents and removal from the carrier, the ceramic tape thus formed can be mechanically stamped to form clay-like, green ceramic layers. A mechanical punch or laser punching operation is used to form via holes in the green layer, and the via holes are subsequently filled with a metal paste. Metal pastes are further patterned on the surface of the green layers by a screen printing process.
A plurality of such green layers is stacked in an aligning fixture and compressed. Under these conditions the thermoplastic component of the green layers can flow and bind the layers together to form a green laminate. A high temperature firing of the green laminate results in a volatilization of the organic components, a densification of the ceramic/metal composite, and subsequent formation of the fired laminate. The densification of the green laminate is accurately controlled so that the desired fired-dimensions are achieved in the final multilayer ceramic device.
Where channels or ducts in the multilayer ceramic are desired, they can be effected by mechanically punching or laser machining openings in the green layers which will occupy internal positions in the laminate. Dubuisson, et al. (U.S. Pat. No. 4,859,520, granted Aug. 22, 1998) discloses the use of such channels for the circulation of a cooling fluid in a monolithic substrate for high power components. Crawford, et al. (U.S. Pat. No. 5,199,165, granted Apr. 6, 1993) discloses a multilayer ceramic with internal channels integral to a heat pipe. Such channels in both patents must be buried deep within the ceramic package if the flatness of the surface is to be maintained. This is due to the nature of the lamination step and the flow of the thermoplastic component of the green layers during compression.
During lamination a compressive stress of the order of 500 psi to 2,000 psi is applied to the green laminate at an elevated temperature of approximately 75.degree. C. The thermoplastic polymer (e.g. polyvinyl butyral) within the green layers flows under these conditions and results in mutual adhesion of the green layers and conformation of the green layers around the pattern of metal paste. In addition to binding the individual green layers into a coherent green laminate, the lamination step determines the density of the green laminate and thus the shrinkage during firing and the dimensional accuracy of the fired laminate. The density of green lamination should be uniform to prevent differential shrinkage during firing.
Vertical cavities in multilayer ceramic modules can be laminated with inserts to transmit the lamination force to the bottom surface of the cavity. Such techniques cannot be used with channels since the layers of the green layers must be aligned and compressed before firing. This means the channels are totally enclosed so inserts cannot be used. It is this compression, along with gravity, which imposes internal stresses in unsupported green layers which are flexible prior to firing. This effect appears greatest where channels are near the surface of the top and bottom layers. The compression and the force of gravity on the outside green layers during lamination result in buckling, which appears as depressions and nonplanar areas, or detrimental nonplanarities, on the outside of the fired laminate above the near-surface channels. These detrimental nonplanarities become especially pronounced with the proximity of the channel to the surface; e.g., when the width of the channel exceeds the thickness of the layer above the channel.
This problem has become worse as the multilayer ceramic devices have been shrunk in size for applications requiring wider channels and thinner layers, and also where the devices are required to support integrated circuit chips which require bonding to flatter and more planar surfaces.
At the same time, new technologies have developed where it is desirable to have near surface channels in multilayer ceramic devices which require high heat resistance for carrying molten solder and thin layers to provide for increased magnetic effect, as disclosed in Tsung Pan, et al. (U.S. Pat. No. 5,779,971, granted Jul. 14, 1998).
In these situations, the sizes of the channels continue to shrink which means that deformations in the top and bottom layers may reduce the flow of fluid in the channels to the extent that the performance of the attached devices are adversely affected and the devices may become inoperable.
A solution to these problems has long been sought but has eluded those skilled in the art for a considerable length of time. Similarly, it has long been known that the problem would become even more severe with advances in technology, and thus a solution has been long sought.